1. Field of the Invention
The present invention generally relates to a stereoscopic antenna for use in connection with a security detector for transmitting a radio signal indicative of detection of the presence of a person and, more particularly, to the stereoscopic antenna capable of exhibiting a uniform pattern of field radiation.
2. Description of the Prior Art
The radiation directional characteristic (directivity) and the field gain are important characteristics of the antenna. The isotropic antenna is capable of radiating energies uniformly in all directions and the curve descriptive of the directivity (directivity diagram) thereof represents a circle in all planes. Requirements for the directivity of the antenna vary depending on the application, but the antenna used in a transmitter of the security detector is desired to be isotropic because the position of the transmitter antenna relative to the receiver varies depending on the place for installation of the security detector.
The rod antenna, an elongated antenna represented by a dipole antenna, is known as a highly efficient antenna capable of exhibiting a high field gain. However, in the case of a micro dipole antenna 100 elongated in a direction z as shown in FIG. 5A, the micro dipole antenna 100 is not isotropic since although the non-directivity is exhibited in the plane perpendicular thereto, that is, in an x-y plane as shown by a line 101 in FIG. 5B, but the 8-shaped directivity is exhibited in a plane containing the micro dipole antenna 100, that is, in the y-z plane as shown by a line 102 in FIG. 5C. Also, in order for the rod antenna to exhibit a high efficiency, it is necessary that the length of the rod antenna as measured in a direction z which conforms to the heightwise direction has to be chosen to be of a value equal to .lambda./2, 3.lambda./8 or .lambda./4 to thereby allow it to resonate at the wavelength .lambda.. This results in the rod antenna having a substantial length and, thus, in difficulty in making it compact.
A loop antenna 110 shown in FIG. 6A is of a design easy to make it compact. The loop antenna 110 is generally used having a perimeter chosen to be of a value equal to the wavelength .lambda.. However, even the loop antenna 110 tends to exhibit the 8-shaped directivity in a y-z plane containing the loop antenna 110 as shown by a line 112 in FIG. 6C, and therefore, the loop antenna is not isotropic. Also, the loop antenna 110 has a field gain smaller than that of the rod antenna.